Control of static electricity can be of great importance in many industrial settings where an uncontrolled electrostatic discharge (ESD) or spark can result in serious damage. For example, static discharges can bring about the destruction of integrated circuits during some stages of their manufacture. In explosive environments such as in grain elevators or in flammable environments such as on oil drilling rigs, in refineries and in solvent-based processes, a static discharge can be extremely dangerous and must be prevented in order to safeguard life and property.
Organic polymeric textile materials used in these settings can be the source of static discharges due to the normally insulative nature of the materials and have a high value of specific resistance, typically on the order of 10.sup.16 ohm cm, unless the materials are altered to prevent build up of electrical charges on their surfaces by permitting charges found on their surfaces to drain in a controlled manner. To control static electrical charges found in textile materials, electrical conductivity of organic polymeric textile material may be increased through application of antistatic finishes to the textile material or through introduction of fibers, which have a degree of electrical conductivity, into the textile material. Other means for controlling static electric charges include such external devices as grounding straps or wires to carry electrical charges found on the textile material to ground.
Antistatic finishes are commonly applied to organic polymeric textile materials either when the organic polymeric textile material is in fiber form or in fabric form. These finishes typically increase ionic conductivity of the surface on which they are applied thereby promoting static dissipation. However, these finishes are typically not as durable as the polymeric textile materials on which they are applied. Use and cleansing of the organic polymeric textile material can remove these finishes from the fabric surface thereby resulting in a loss of the organic polymeric textile material's ability to dissipate static electric charges found on their surfaces.
Coatings of metals or of conductive carbon may be added to the outside surface of fibers used in producing organic polymeric textile material. However, if the coating used is not as flexible as the fiber on which it is placed, flexing of the fiber while in use may cause cracking in the coating and therefor interruptions in the conductive pathway formed by the coating with a resulting loss of conductivity in the organic polymeric textile material.
Introduction of fibers produced of materials that have a degree of electrical conductivity into the organic polymeric textile material allows a more durable method of altering the surface conductivity of the material. Certain types of carbon and metal fibers are inherently conductive, and incorporation of these fibers even at a small percentage of the total fiber content of the textile material into anorganic polymeric textile material can increase the conductivity of the textile material to the extent that a previously static-prone material can be rendered static dissipative.
However, use of these fibers presents additional problems. Carbon fibers are relatively brittle when compared to the majority of fibers presently in use and exhibit the tendency to break when flexed. If the textile material is used in a high efficiency particle filter in a clean environment, breakage of carbon fibers results in a reduction in the static dissipative capabilities of the organic polymeric textile material as well as providing a source of contamination to the environment which is known as media migration. This contamination can be especially burdensome if the organic polymeric textile material is being used in a clean room for the production of integrated circuits or for the assembly of highly critical optical parts.
Use of metal fibers, usually stainless steel, also presents problems. If the textile material is incorporated into a filter structure, metal fibers may be lost through media migration from the textile material and contaminate the collected product and/or the filtered stream. This would pose a significant downstream problem to such industries as food processors, pharmaceutical manufactures and chemical producers.
Metal fibers also present a problem for those who process textile materials. Processors typically utilize metal detectors in their equipment to detect the presence of stray pieces of metal commonly known in the art as "tramp metal". These stra pieces of metal if undetected can result in a flawed textile material as well as posing a hazard to equipment and personnel that would subsequently come in contact with the textile material. Use of metal fibers precludes the use of these detectors when producing an organic polymeric textile material containing these metal fibers.
Organic polymeric fibers commonly used to produce organic polymeric textile materials, such as polyamides and polyesters, have had conductive material incorporated into their structures to impart a degree of conductivity to these materials. The conductive material may be found in a stripe of conductive material on the outside of the fiber, a conductive core of material found in the fiber or an amount of conductive material dispersed throughout the fiber's structure. However, these materials have a relatively limited usable range with respect to the temperatures and chemical exposures these materials can withstand without degrading. In certain areas where organic polymeric textile materials are used for example as filter media, the need for chemical and temperature resistance is great, and the need for a flexible, electrically stable, static dissipative textile material is critical.
Polytetrafluoroethylene (PTFE) is an organic polymeric material that is highly resistant to many corrosive chemicals and remains stable over a wide range of temperatures. PTFE, and especially a particular high strength form of PTFE, expanded porous polytetrafluoroethylene, (ePTFE), has demonstrated utility as an organic polymeric textile material for use in demanding environments. However, ePTFE is also an excellent insulator and is commonly used as an insulating layer in electronic cable constructions.
Static dissipative textile materials that are highly resistant to corrosive attack while remaining electrically stable and flexible over a wide range of temperatures is the object of the present invention.